This invention is especially useful with respect to poly(ethylene terephthalate) (PET) which is widely used in the food-packaging industry. Worldwide usage for beverage bottles alone amounts to well over one billion pounds annually.
When PET is used to package foods and beverages, the presence of acetaldehyde in the polymer above certain concentration levels (above approximately 10 ppm) imparts undesirable flavors to the packaged food and beverages. This problem is eliminated successfully in the conventional method of producing high molecular weight PET, which involves the melt-phase polymerization of either dimethyl terephthalate (DMT) or terephthalic acid (TPA) with ethylene glycol to produce PET with an inherent viscosity (I.V.) of about 0.6, conversion of the molten PET to pellets, and solid-state polymerizing these pellets at temperatures of approximately 190.degree.-230.degree. C. for approximately 4-16 hours to produce PET of the required I.V. for food-packaging use. This latter step of heating the PET pellets under controlled conditions is an additional polymerization step and is referred to as solid stating. It is a fortunate feature of this solid state polymerization that it removes most of the acetaldehyde from the PET, as it is a characteristic of the PET polymer pellets produced by conventional melt-phase polymerization and pelletization that it contain relatively high levels of acetaldehyde.
The solid stating process, while effective, is both time-consuming and costly, and it would obviously be desirable to eliminate it. The subject invention provides a process for acetaldehyde reduction in molten polymer without the need for solid-state acetaldehyde removal processes. Savings in time and cost should be significant. Furthermore, the invention provides an improved method for removing acetaldehyde from molten PET which might be obtained from remelting pellets which have an undesirable level of acetaldehyde, or for removing acetaldehyde generated in the solid polymer remelting process.
Melt phase ventilation is known and practiced for devolatilization of polymer and polyester melts. Examples exist which show that vacuum devolatilization reduces the acetaldehyde levels in molten polyester. However vacuum systems are often difficult to operate and maintain at the necessary high vacuum levels needed for acetaldehyde devolatilization. Furthermore, air-leak free operation of vacuum vented extruders is often difficult.
Using the process described in this current invention, polyesters with the proper I.V. and desirably low levels of acetaldehyde can be provided without the need for acetaldehyde removal in the solid state.
Typically, acetaldehyde is removed from polyesters in the solid phase. For example, U.S. Pat. No. 4,263,425 describes a solid state process for eliminating acetaldehyde from polyester chips. The author acknowledges that acetaldehyde can be partially eliminated from a polyester melt using vacuum, but states that tolerable levels cannot be achieved. Furthermore, he states that such a process would not be desirable seeing that elimination of acetaldehyde from a polyester melt at higher viscosities is even more difficult. Therefore, the author states that it is necessary to eliminate acetaldehyde in the solid state for acceptable food packaging products.
U.S. Pat. No. 4,064,112 describes a method for overcoming sticking problems during the solid stating process. It discusses the disadvantages of a solely melt phase process and states that "elevated concentrations of acetaldehyde are to be expected in the melt".
U.S. Pat. No. 5,102,594 describes the crystallization of solid PET in a vented extruder under vacuum to reduce acetaldehyde content and build up the molecular weight of the polymer. The devolatilized solid polymer is immediately melted and extruded directly into a final product.
U.S. Pat. No. 4,591,629 describes a process for continuous production of high molecular weight polyesters in a two-stage process for treating polyester in the solid phase in which (1) in a first stage the polyester is treated with steam or a purging agent or air containing steam at a temperature of 100.degree.-245.degree. C. and (2) in a second stage it is post-condensed at 200.degree.-245.degree. C. with a purging agent and/or air at normal pressure or under vacuum. The process is said to be especially useful for the production of high molecular weight PET having a total content of dissolved and bound acetaldehyde of less than 3 ppm. This PET is said to especially useful for the production of bottles and other containers for foodstuffs.
Using extruders to devolatilize polymer melt streams is known in the literature. For instance, Mack [M. H. Mack, Plastics Engineering, pp 47-51 (July 1986)] discusses some selection criteria for a variety of melt devolatilization applications. For single screw extruders, Mack's work shows devolatilization to the range of 5 ppm residual ethylene in low viscosity ethylene/vinyl acetate copolymers. In higher viscosity polymers, 15 ppm levels were all that was achievable. Biesenberger, et al., has published both theoretical and experimental data on devolatilization of styrene from polystyrene melts in single screw vented extruders under vacuum and purging agent blankets, as well as other examples. For instance, Biesenberger's data and examples show residual styrene monomer reducing from over 5000 ppm to approximately 100 ppm after devolatilization. Biesenberger compares vacuum venting and nitrogen purge venting at atmospheric pressure and concludes that vacuum venting is more efficient than agent purging for removing volatiles from polymer melts. See J. A. Biesenberger and G. Kessidis, Polymer Engineering Science, 22, 13, pp 832-836 (1982) and J. A. Biesenberger and D. H. Sebastian, Principles of Polymerization Engineering, Krieger Publishing Company, Chapter 6 (Malabar, Fla., 1983).
Prior patent art teaches that vacuum devolatilization is a viable way to reduce or remove volatiles from a molten polymer. For example, U.S. Pat. No. 4,362,852 describes a process for devolatilizing molten polyester and polyamide with a rotary disk processor operating under vacuum. The process is stated to lower residual monomers in polyamides to 2.5 weight percent. The process is also stated to lower ethylene carbonate and carbon dioxide in polyesters to levels of 100 ppm and 50 ppm, respectively.
U.S. Pat. No. 4,980,105 provides an example of an extruder process for devolatilizing a polymer melt to remove by-products. In this case, a by-product was formed in an earlier reaction step and remains in the polymer. Extruder devolatilization using vacuum venting removed this by-product. However, devolatilization of acetaldehyde is substantially different from this example since acetaldehyde is a continuously produced by-product in molten polyesters.
Japanese Kokai Patent Application No. Sho 53[1978]71162 describes a method of melt processing polyester and lowering acetaldehyde content by holding remolten PET at pressures less than 250 mm Hg for at least 5 seconds and then under ordinary pressure or increased pressure for less than 5 minutes. Cited examples in this process demonstrate that the removal efficiency of acetaldehyde increases as the vacuum pressure is lowered in an extruder port. The authors state that at pressures above 250 torr and at long venting times, acetaldehyde is difficult to remove from polyesters.
U.S. Pat. No. 4,230,819 describes the removal of acetaldehyde from crystalline PET with a dry agent (air or nitrogen at 170.degree.-250.degree. C.). It states that acetaldehyde cannot be completely removed from PET by heating it under pressure.
U.S. Pat. No. 4,255,295 describes a process for production of a polymer of good spinnability from waste. It consists of compressing the finely-chopped waste by means of a screw until a bulk density of 500 kg/m.sup.3 is reached, introducing same into a double-screw degassing extruder where it is melted, and subjecting the molten polymer to a post-condensation operation under reduced pressure. During the melting of the polymer and the post-condensation operation, traces of water and volatile impurities are removed. The polymer is said to be suitable for use in spinning operations and in plastics, but only nonwoven webs are cited.
U.S. Pat. No. 4,142,040 discloses a method of processing in the molten state a saturated polyester resin so as to minimize degradation to yield acetaldehyde. This patent discloses in column 4, lines 38 et seq., "inert gas--is introduced through one or more conduits 3 into the bottom of the hopper or through one or more conduits 3a into the feeding zone (or both). The inert gas flushes essentially all air from the polyester as it advances through the initial part of the feeding zone."